Conventionally, a data transmission system employs a Nyquist filter whose reply for an impulse is zero (0) for every interval T just before and after a peak (T: data transmission interval). As replies of adjacent data are zero on a sample point of other sampled data, interference between codes can be prevented.
To function as above, a Nyquist filter whose amplitude becomes half at the point where Nyquist frequency f.sub.N (=1/2T) is used, and it is odd-symmetrically rolled off with a squared cosine around the half amplitude point in the cutoff area. In this case the phase characteristics is not considered, or is designed to have linear phase characteristics usually.
Instead of a simple Nyquist filter, low-pass filters (root Nyquist filters) having equal amplitude are set for transmission and reception and their characteristics are made the same as that of the Nyquist filter above. In this case, the root Nyquist filter must have cosine characteristics that can be made the squared cosine characteristics above.